The following terms are used throughout the specification with the meanings and in the context described.
Digital item—a digital object containing structure, metadata and resources. Digital items may be declared using the Digital Item Declaration Language (DIDL), typically a DIDL specified by the MPEG standard, however, it will be appreciated the invention is not limited to digital item defined by such a DIDL and other definitions are equally applicable.
Content digital item—a digital item created for the purpose of allowing the distribution of content including multimedia files, documents and the like.
Context digital item—a digital item created for purpose of capturing the session state of a content digital item. A context digital item may include information relating to the current configuration of a content digital item, the user preferences used to configure a content digital item, the terminal capabilities of the device upon which the content digital item was configured.
Situations may arise where a user who is using a digital item on a device may want to use the digital item on a different device and yet maintain its state.
As an example, consider the following scenario:
A user has a location aware multimedia system within their house. Each device within the system has the capability to render digital items. Devices within the system include: televisions and stereos (different stereos in different rooms).
The user requests a digital item that represents a music album. The digital item contains multiple soundtracks, as well as associated video clips. The user initially starts viewing the digital item on their television. However, they also want the capability to walk through the house and listen to the digital item wherever they are.
As the user wanders through the house the digital item must be transported to the device that is in the room closest to the user. When more than one device exists within the room the user may express a preference as to which device is used to render the digital item.
To allow the user to receive an uninterrupted listening experience the digital item and its current state (i.e: which track is playing and the position within the track) must be transported between devices when a handover occurs.
To allow transportation of a digital item between rendering devices to occur transparently to the user the current state of the user's session must be preserved. In this context the user's session refers to: the digital item's configuration and its current rendering state. The digital item declaration language (DIDL) provides tools that may be used to achieve this goal.
The DIDL provides a set of elements that allow the preservation of session state.
These elements include:                Annotation: this element allows an end user to associate descriptors, Assertions, and Anchors with a Digital Item without affecting the Digital Item's signature;        Descriptors allow the inclusion of meta data from any schema allowing applications to add descriptions of their current state;        Assertions: this element allows the predicates within a choice to be asserted as true or false. An end user may add Assertions (using Annotations) to a digital item to preserve the state of the selections made within choices.        Anchors: this element allows the creation of bookmarks within media resources. Anchors could thus be used to record the current status of a tracks playback.        